Research has shown that implantation of congenitally deaf children accelerates their development of speech-based communication. Theories of speech and language development predict that earlier implantation should provide improved benefits for speech and language development, and also, that growth in speech and language development should be constrained as the child approaches adolescence. The first longitudinal study will evaluate several measures of speech, language, and auditory perception as indices of benefit from hearing aid use during infancy. It is our goal that these measures may ultimately help determine implant candidacy. Two additional longitudinal studies are proposed that will test whether there are constraints on development that can be shown among children with cochlear implants. One study will follow children implanted between 12- and 36- months old for 24 months. During this time period we plan to monitor their speech, language, and auditory perceptual development in order to determine if earlier implantation affects growth rates and 24-month, post-stimulation, achievement levels. The second longitudinal study concerns children and adolescents with at least 10 years of implant experience. We plan to examine growth in speech and language from 5 to at least 10 years post-stimulation; we will also examine achievement at 10 years post-stimulation to determine if there are limits to long-term growth. We will also examine the extent to which long-term achievement levels approach levels found in normal-hearing adolescents and adults. The benefits of implantation on speech production and language development also result in a prediction that there will be subsequent gains in reading and general scholastic achievement among these children. Early reading achievement in second grade and later reading and scholastic achievement in 10th grade will be examined in two cross-sectional studies. Finally, achievement levels of speech and language development among implant users appears to exceed that found in many children with more acoustic hearing who wear hearing aids. A cross-sectional design will compare school-age implant users with age-mates who are hearing aid users. It is predicted that implant users will have better speech and language than hearing aid users with low audibility levels.